An experimental and simulation study of fuel consumption and NOx emissions from biofueled diesel engines
Alternative fuel vehicles are gaining importance as a means of reducing petroleum dependence. The use of biodiesel, which is a renewable diesel fuel produced from plant or animal fats, has several advantages. It significantly reduces carbon dioxide, carbon monoxide, unburned hydrocarbon and particulate matter emissions. However biodiesel combustion also typically results in increased fuel consumption and smog-generating nitrogen oxide (NO x) emissions relative to conventional petroleum diesel. In order to determine the cause of, and ultimately develop mitigation strategies for, increased biodiesel combustion fuel consumption and NOx emissions, a simulation model was developed and validated. More specifically, this thesis outlines the extension of a previously developed whole engine model to capture biodiesel combustion effects that were observed in experiments - namely, increases in brake specific fuel consumption (BSFC) and NOx, and reductions in torque and power. The previously developed model which incorporates a multi-zone, quasi-dimensional combustion sub-model for a 6.7 liter six cylinder Cummins 2007 diesel engine with cooled exhaust gas recirculation, variable geometry turbocharging and common rail fuel injection has previously been validated for conventional diesel combustion at 22 different engine operating conditions with excellent predictions for in-cylinder pressure, heat release rate, NOx, and gas exchange. In order to implement biodiesel combustion in the model, only the key fuel properties were modified. Biodiesel has a different molecular composition (number of carbon, hydrogen and oxygen atoms), density, heat of vaporization, distillation temperatures and lower heating value compared to conventional diesel. These properties, along with changes in ignition delay characteristics, were implemented to reflect soy-biodiesel fuel. The average model predictions of biodiesel combustion at three very different operating conditions for torque, indicated mean effective pressure (IMEP) and BSFC are within 8%, 6% and 9% respectively of the biodiesel experimental results from the 2007 Cummins engine at Purdue. The NOx emission prediction for biodiesel combustion is within 15% of experimental data. The experimental increase in NO x and BSFC from conventional diesel to biodiesel for the three points was 19%, 17% and 38% for NOx and 11%, 13% and 11% for BSFC. Simulation model predictions are consistent with this, showing NOx increases of 22%, 19% and 44% and BSFC increases of 16%, 11% and 11% at the corresponding operating points. These results indicate that the model effectively captures the increasing NOx and fuel consumption trends and can be used to investigate the cause of, and mitigation strategies for, these increases. The model predictions for local properties in the flame front zone were studied and a new theory was proposed for the cause of increases in NOx emissions with biodiesel combustion. It was observed that the biodiesel equivalence ratio at the flame front is closer to stoichiometric for most part of the burn duration, while that of diesel is much higher. Higher equivalence ratio for diesel means more fuel present at the flame front which absorbs heat and reduces the temperature in the burn zone. Hence biodiesel burn zone temperatures are higher as compared to diesel, which leads to higher NOxformation rates for biodiesel.^
Gregory M. Shaver, Purdue University.